Enlarge /. Artist's impression of a large ring system that surrounds an exoplanet.
Many of the exoplanets we've discovered look at least vaguely like something we're familiar with. Exoplanets have been described as super earths, mini-neptunes, hot Jupiter, etc. But not everything is completely familiar, and we came across a number of strange balls. Under these strange spheres is a group of extremely low density planets. The lack of an analogy to the solar system forced us to come up with a non-planetary nickname: the super puffs.
Many of the super puffs are somewhat difficult to explain with normal planetary physics. However, a group of researchers based in Europe was looking for a possible alternative explanation for a super puff: it is a normal planet with unusual rings. The answer they found is that we can't really say it right now, but they're suggesting ways that we could possibly sort it all out.
More than just a train
Although strong cores make the cores of gas giants fairly dense on average, they are still gas and often have a large percentage of lighter gases such as hydrogen and helium. As is well known, this has led to the claim that Saturn would hover if you could somehow drop it on some water. (It would actually be torn to pieces by the gravity of so much water, but we mostly let this detail slide.)
Super puffs make Saturn look like a heavyweight. The water resistance is one gram per cubic centimeter. The Saturn density is 0.7 g / cm3. A super puff can have a density close to 0.1 g / cm³.
And that creates some physical problems. At this point the matter becomes very diffuse and it is difficult to find ways to have enough mass to hold everything together. There are some planets that are close enough to their host stars that they are likely to have warmed up and expanded their atmosphere accordingly. This is not stable – these planets will likely lose their atmosphere over time – but the process is slow enough that we can probably catch them in the middle.
Another possible explanation is a combination of errors on our part. The density of something is simply the mass of the object divided by the volume it occupies. We determine the mass of a planet through its gravitational interactions with its host star. We determine the volume by looking at how much light the planet blocks when it passes in front of its host star. There are errors in both and the potential for complications like planets elsewhere in the exosolar system that we have not yet identified. It is possible that some of the super puffs are a little closer to normal planets, and we were just a little bit off our measurements.
Inflated at the HIP
The new work focuses on a Super Puff, HIP 41378 f, a planet with extremely low density and Saturn size. It is an interesting super puff because at least one explanation for its low density can be excluded: the planet orbits its star at a greater distance than the earth orbits our sun (it takes 542 days to complete an orbit). While HIP 41378 f is a little brighter than the sun, it is nowhere near bright enough to inflate an atmosphere at this distance.
However, the measurement problem could easily be a problem since we have detailed data from only two transits. Although there is much more data on the mass of the planet, it is possible that we are a little off in volume calculations. However, the researchers take the values we have and try to calculate the relative likelihood of two scenarios: one that is an extremely strange planet and the other that it is a smaller planet with rings .
These probabilities are a bit wavy as all options seem pretty unlikely. To get a super puff planet like this, a dense core must be surrounded by an extremely weak atmosphere. But an atmosphere that is thin will not be opaque enough to block out as much starlight as HIP 41378 f. So you have to assume that there is a process that increases opacity. Clouds don't because there is probably not enough material in the atmosphere to form clouds. So you have to imagine a process in which dust gets from the surface of the planet into the upper atmosphere – which has to be extremely far from the surface of the planet to get something so big.
All of this makes a single spherical planet rather unlikely, which in your opinion makes things cheap for a ringed planet. But that's only because we didn't deal with the issues there.
Put a ring on it
For the ring to work, it must be in a plane that does not coincide with the planet on the planetary orbit. This is certainly possible, but this alignment would mean that the gap between the planet and the ring should be aligned so that the starlight is transmitted at different points during its orbit. This should result in abnormal light leaks in the middle of the transit. To the best of the researchers' knowledge, none of them were there. But with just two transits worth of data, it would have been difficult to recognize them anyway.
However, this requires an essentially impossible situation: the rings must be dense enough to minimize the passage of light and, for the same reason, extend almost to the surface of the planet. These two situations are largely incompatible.
The only thing that looks good on the ring model is that the radius of the planet drops from nine times the Earth's radius to just 3.7 times. This increases its density to higher than that of Saturn and higher than water and places it near Uranus ’. That's pretty reasonable, at least as far as the physics of the planet itself is concerned.
In the end, the researchers conclude that there is currently insufficient data to choose one of the two options, and even if they try to do probability calculations, they will have to make many assumptions that are not really justified by the current data. Your solution, of course, is to get more data. They find that further observations to limit the mass of the planet are already planned.
However, they suggest that the most interesting thing we can do is to observe a transit across the far infrared end of the spectrum. In this area, the photons should largely come directly through the dusty material in the disk. So if a plate is responsible here, the transit should be greatly reduced compared to its depth at other wavelengths. Given that there are more than one and a half years between the transits, we unfortunately have to be patient.
Astronomy and Astrophysics, 2020. DOI: 10.1051 / 0004-6361 / 202037618 (About DOIs).